In general, a reference voltage is widely used in various analog circuits. A circuit for generating the reference voltage includes a band gap regulator for suppressing a fluctuation in an output voltage due to a change in a source voltage (for example, see Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 6-309052
FIG. 1 is a diagram showing a structure of a conventional reference voltage generation circuit utilizing a band gap regulator. A band gap regulator 10 shown in FIG. 1 has a current determination circuit 11 including a positive feedback circuit, a current mirror circuit 12, and a voltage generation circuit 13 connected through the current mirror circuit 12 to generate a voltage upon receipt of a current determined by the current determination circuit 11.
The current determination circuit 11 is constituted by PNP transistors Q1 and Q2, Nch transistors M1 and M2, Pch transistors M3 and M4, and a resistor R1. The PNP transistor Q1 has a collector and a base connected to a ground and has an emitter connected to a source of the Nch transistor M1. A gate of the Nch transistor M1 is diode connected to its own drain and is connected to a gate of the Nch transistor M2.
The drain of the Nch transistor M1 is also connected to a drain of the Pch transistor M3. On the other hand, the Nch transistor M2 has a drain connected to that of the Pch transistor M4 and has a source connected to an emitter of the PNP transistor Q2 through the resistor R1. A collector and a base of the PNP transistor Q2 are connected to grounds. A gate of the Pch transistor M4 is diode connected to its own drain and is connected to a gate of the Pch transistor M3.
The current mirror circuit 12 is constituted by connecting the gates of the Pch transistors M3, M4 and M5 in common and diode connecting the gate of the Pch transistor M4 to its own drain. The Pch transistors M3, M4 and M5 have sources connected to a power supply VOD.
The voltage generation circuit 13 is constituted by a PNP transistor Q3, the Pch transistor M5 and a resistor R2. The Pch transistor M5 has a drain connected to an emitter of the PNP transistor Q3 through the resistor R2. The PNP transistor Q3 has a collector and a base connected to grounds.
An operational amplifier 14 has a positive terminal connected between the drain of the Pch transistor M5 and the resistor R2. An output terminal Vout having a reference voltage is provided on an output side of the operational amplifier 14 and voltage division resistors R3 and R4 are provided between the output terminal Vout and a ground, and a divided output voltage is negative fed back to a negative terminal of the operational amplifier 14.
In the band gap regulator 10 described above, a positive feedback is applied to the Nch transistor M1 and M2 portions, and impedances of the transistors Q1 and Q2 and the resistors R1 and R2 act as noises to influence an output signal when they are positive fed back. A noise voltage (a thermal noise) appearing in the resistors R1 and R2 is amplified by the positive feedback to have a great value. For this reason, an output impedance of the band gap regulator 10 has a very great value and an output current thereof has a small value.
In order to amplify the small output current, the operational amplifier 14 is provided on an output side of the band gap regulator 10. In the operational amplifier 14, an input conversion noise voltage is generated. A noise on the output side of the operational amplifier 14 has a value obtained by multiplying the input conversion noise voltage by an amplification factor of the operational amplifier 14. When the amplification factor is great, the output noise (the thermal noise) of the operational amplifier 14 also has such a great value as not to be disregarded.